1. Field of the Invention
This invention is in the field of gel coat compositions, components for making gel coat compositions, processes for making gel coat compositions for use in making gel coated articles, processes for making gel coated articles, and gel coated articles.
2. Description of the Relevant Art
Durable polymeric coatings on composite materials known as gel coats are well known. They are typically found on composite materials that are exposed to the elements or that require a smooth glossy finish. Examples of items having gel coats include boat hulls, jet skis, pools, spas, composite body panels on automobiles, vans and trucks, and the like.
Such gel coats are often formed by applying a gel coat composition to the inside of an open mold, applying the composite article to the gel coat composition so that it is in adhering contact with the gel coat composition, curing the gel coat, and removing the gel coated article from the mold. Gel coated articles can also be made by forming a composite material in a multi-part mold, opening the mold sufficiently to inject or apply a gel coat composition, closing the mold, curing the gel coat, and removing the gel coated article from the mold.
Gel coat compositions are known which comprise oil-free unsaturated polyester condensation polymers in mixtures of unsaturated reactive diluents, such as styrene, which form crosslinked polymers by using a polymerization initiator, such as a peroxide. A limitation of gel coats made from unsaturated polyesters is that they typically have less than desired hydrolytic stability and durability. The hydrolytic instability can be attributed to polyol acidity and the presence of ester groups and the lack of exterior durability can be attributed to the presence of the aromatic component which is used as the reactive diluent.
Several approaches for addressing these limitations have been described in the prior art. These approaches include making polyester modifications and polyester alloys such as described in U.S. Pat. Nos. 4,587,323; 5,118,783; and 5,376,460 and in SPI Composites Institute Paper 17-H (1983); using epoxy chemistry such as described in U.S. Pat. Nos. 4,367,192 and 5,389,443 and in SPI Composites Institute Paper 15-C (1991); urethane/urea chemistry such as described in U.S. Pat. Nos. 3,928,299; 4,374,238 and 5,387,750; European Patent 254,232; and Plastics Technology, Vol. 34, No. 3, March 1988, page 13; and acrylic chemistry as described in U.S. Pat. Nos. 4,177,338; 4,742,121; and 5,045,613.
While these approaches have lead to improvements in hydrolytic stability and durability, room exists for further improvements in these characteristics. In addition, improvements are still needed in gel coat composition rheology. The desirable gel coat composition rheology has a high thixotropic ratio to allow for ease of application to the mold while preventing sagging or running after it is applied, particularly in open mold coating processes.
These and other desirable objectives are achieved by gel coat compositions, components for making gel coat compositions, processes for making a gel coat composition for use in making gel coated articles, processes for making gel coated articles and gel coated articles of the present invention.
One aspect of the present invention is a polymer for making gel coat compositions represented by the formula: 
wherein
X is a divalent saturated aliphatic group having a pendant ester group without any polymerizable double bonds;
Y is a divalent saturated aliphatic group having a pendant group Z;
Z is a group of the formula:
xe2x80x94COxe2x80x94Oxe2x80x94R1xe2x80x94Oxe2x80x94COxe2x80x94R2xe2x80x83xe2x80x83(IA)
in which R1 is an aliphatic, saturated, hydroxyl-containing group and R2 is a hydrocarbyl group with at least one polymerizable carbonxe2x80x94carbon double bond;
W is a group of the formula R3xe2x80x94Sxe2x80x94 in which R3 is a hydrocarbyl group having 2 to 14 carbon atoms and not having a polymerizable carbonxe2x80x94carbon double bond;
m is a number in the range from 1 to 10;
n is a number in the range from 1 to 50;
n/m is in the range from about 1 to 5; and the polymer has a number average molecular weight of at least 2,500.
Another aspect of this invention is a process for making a polymer for use in a gel coat composition comprising:
(a) reacting at least one monomer of the formula: 
xe2x80x83in which R4 is a hydrogen atom or a saturated aliphatic group of 1 to 8 carbon atoms and R1 is a saturated aliphatic group of 1 to 8 carbon atoms with at least one monomer of the formula: 
xe2x80x83in which R6 is a hydrogen atom or a saturated aliphatic group of 1 to 8 carbon atoms R1 represents a divalent, aliphatic, saturated, hydroxyl-containing group having at least one carbon atom and the molar ratio of the compound of formula (II) to the compound of formula (III) is in the range from about 1:1 to about 5:1;
(b) adding a polymerization initiator;
(c) adding a polymer chain terminating compound of the formula
R3xe2x80x94SH
xe2x80x83in which R3 is a hydrocarbyl group having at least about 2 carbon atoms and not having a polymerizable carbonxe2x80x94carbon double bond, such that the polymer produced in step (a) will have a number average molecular weight of at least 2,500 after completing step (b); and
(d) reacting the product of step (b) with a monomer of the formula: 
xe2x80x83in which R7 is either xe2x80x94H or xe2x80x94CH3, in an oxirane ring-opening reaction.
Yet another aspect of this invention is a gel coat composition comprising:
(a) at least one polymer for making a gel coat composition described above and
(b) a rheology synergist concentrate.
Yet another aspect of this invention is a process for making a gel coat comprising:
(a) dispersing in the polymer described above a rheology synergist concentrate, a free radical initiator and, optionally, a filler; and
(b) adding as needed an acrylic monomer until a predetermined viscosity is obtained.
Another aspect of this invention is a rheology synergist concentrate for making a gel coat composition comprising:
(a) at least one thixotrope;
(b) at least one aliphatic alcohol having from 1 to 20 carbon atoms and 1 to 3 hydroxy groups; and
(c) at least one alkali metal salt of an organic acid.
Yet another aspect of this invention is a process for making a gel coated article comprising:
(a) applying to a mold the gel coat composition described above;
(b) partially curing the gel coat of step (a);
(c) applying to the exposed surface of the gel coat of step (b) an article having a shape corresponding to the shape of the gel coat in the mold, to form a laminate;
(d) allowing the laminate of step (c) to further cure to form the gel coated article; and
(e) removing the gel coated article of step (d) from the mold.
Another aspect of this invention is the gel coated article obtainable from the polymers, compositions, and processes of the present invention described above.